Electronic structures of mixed sandwich <Emphasis Type="Italic">ansa</Emphasis>-complexes of chromium as studied by gas-phase absorption spectroscopy and quantum chemistry

Gas-phase electronic absorption spectra of ansa-(cycloheptatrienylcyclopentadienyl)-chromium complexes with silicon- and germanium-containing bridges were recorded for the first time. The spectrum of the —(SiMe₂)₂— bridged compound exhibits a Rydberg structure arising from the transitions originating at HOMO. Shortening of the bridge causes the Rydberg bands to broaden beyond detection. Density functional calculations were performed to investigate the electron density distribution in neutral complexes and the corresponding cations. An increase in the angle between the carbocycles on going from the —(SiMe₂)₂— bridged com pound to the —SiMe₂— and —GeMe₂— bridged complexes was shown to cause substantial changes in the composition and energies of the HOMO and Rydberg orbitals. Delocalization of the HOMO in the —SiMe₂— and —GeMe₂— bridged molecules leads to broadening of the Rydberg bands in the gas-phase spectra.